The present invention relates generally to inkjet printing. More particularly, the present invention relates to changing the distribution characteristics of ink exiting the print head.
Thermal inkjet printers operate by expelling a small volume of ink through a plurality of small nozzles or orifices in a print head surface that is in proximity to a printable medium. The nozzles are arranged in the surface of the print head such that the expulsion of a droplet of ink from a predetermined number of nozzles relative to a particular position of the print medium results in the printing of a portion of a desired character or image. Controlled repositioning of the print medium and/or printhead and another expulsion of ink droplets continues the production of more pixels of the desired character or image. Inks of selected colors may be coupled to individual arrangements of nozzles so that the selected firing of orifices can produce a multicolored image by the inkjet printer.
Expulsion of the ink droplet in a conventional thermal inkjet printer is a result of rapid thermal heating of the ink to a temperature that exceeds the boiling point of the ink solvent. The heating creates a gas-phase bubble of ink. Each nozzle is coupled to a small, unique ink firing chamber filled with ink that has an individually addressable heating element thermally coupled to the ink. This heating element is typically a resistor. As the bubble nucleates and expands, it displaces a volume of ink that is forced out of the nozzle and deposited on the print medium. The bubble then collapses and the displaced volume of ink is replenished from a larger ink reservoir by way of the ink feed channels.
The superheat temperature of the ink is the temperature at which the liquid ink undergoes a phase change from a liquid state to a gaseous state. The inks used in typical thermal inkjet printers have a superheat temperature in the range of 250xc2x0 C. to 300xc2x0 C.
After the deactivation of the heater resistor and the expulsion of ink from the firing chamber, ink flows back into the firing chamber to fill the volume vacated by the ink that was expelled. It is desirable to have the ink refill the chamber as quickly as possible to enable rapid firing of the nozzles of the print head. The faster the nozzles can fire, the faster the print speed that can be obtained.
Inks used in these types of print heads must have certain desirable characteristics. For example, inks that have a high decel characteristic cannot be used. Decel is the loss of drop velocity and weight that occurs during high speed firing of the print head. Inks that have a high decel reduce the print quality because of the misdirection and low drop weight of the ink drops. This is one cause of banding in the print output as well as other quality problems.
Various methods for improving the ink decel characteristic have been tried. For example, adding or removing components have shown improvements. However, changing the ink formulation in this manner compromises desirable ink properties such as ink stability and performance. Limited selections of inks, therefore, are available for inkjet use. There is a resulting unforeseen need to be able to use a larger range of inks in inkjet printers while still producing a high quality print output.
The present invention encompasses a process for reducing a decel characteristic of ink. The ink is part of an inkjet pen device that has a print head. The print head comprises heating elements that are coupled to the ink such that the electrical heating of the heating element causes the ink""s temperature to increase.
When the printer receives a print command from a computer apparatus, the printer""s controller generates a single, short, electrical pulse to the heating element. In the preferred embodiment, this pulse is less than 1 microsecond. In one embodiment, the pulse is in the range of 0.20-0.60 microseconds and occurs more than 1.00 microsecond prior to the product main drive pulse.